At 23, Sabine Lisicki has been around long enough to know how to handle facing a tennis opponent for the first time.

What works best for the 40th-ranked WTA player is to adjust on the fly. Facing 18-year-old Madison Keys for the first time Tuesday in their first-round match in the U.S. National Indoor Tennis Championships, the No. 3-seeded Lisicki spent the first set feeling Keys out. By the end of the set, Lisicki’s adjustments allowed her to put away Keys, 7-5, 6-3.

“If you don’t know how someone plays, you just have to go in and see what happens and then adjust to it,” Lisicki said. “In the beginning, I didn’t handle her second serve well. Later on, toward then end of the first set, I played pretty decent.”

Joining Lisicki in the second round is Andrea Hlavackova, who defeated Melanie Oudin, 6-3, 3-6, 6-3, in a match that lasted just over two hours. No. 5 seed Lucie Hradecka was upset by qualifier Claire Feuerstein, 6-3, 6-4, while Jamie Hampton topped Coco Vandeweghe, 6-3, 6-0 in their second-round match.

Early on, Keys caught Lisicki off guard with her serve. Keys scored points on her first 12 serves that helped her jump out to a 4-2 lead in the first set.

“I think I was staying pretty calm,” said Keys, a qualifier. “I had a good idea of what I wanted to do. Everything was working and I was serving well.”

But by her fifth service game, Lisicki had figured out Keys’ serve. With Keys leading 5-4 and serving for the set, Lisicki won four straight points to break serve.

She then closed out the set by holding serve and breaking Keys again.

“That was an important point in the match,” Lisicki said of breaking Keys the first time. “I could turn the match around. When you’re serving for the set, you want to get your service game through. I think it was good of me to break her at that point. It gave me a lot of confidence.”

With her confidence high, Lisicki began unleashing her big first serve, which topped out at 124 mph.

“I felt more comfortable,” she said. “When I play well, it’s hard for a lot of people to play their game. I’m an aggressive player. It’s not easy to play me.”

In her first round match, qualifier Feuerstein, 26, was on top of her service game, particularly in the first set. In her previous three matches against Hradecka, Feuerstein said she put too much pressure on her serve.

This time she took a different approach.

“For this match, I tried to be relaxed and take my time and I served very well today,” Feuerstein said. “I think I won the match with my serve.”

Hradecka took a 2-0 lead in the second set, but was unable to capitalize on her next service game.

“I had her 2-0 and was serving and I had three or four double faults,” she said. “Maybe if I had won that game, things would have turned on my side.”

Oudin fell behind 4-0 in her opening set against Hlavackova, a deficit that proved difficult to overcome. She bounced back to pull out the second set, but Hlavackova regrouped to advance to the second round.